The abundant, low-cost production of potent fuels, which can be used in intrinsically clean energy processes, i.e. processes which do not produce and emit greenhouse gases and other pollutants is a challenging task.
Steam reforming is generally used to produce hydrogen from hydrocarbons. Steam reforming of natural gas, sometimes referred to as steam methane reforming (SMR), is the most common method of producing commercial bulk hydrogen, as well as hydrogen, used in the industrial synthesis of ammonia.
The steam reforming of methane and other hydrocarbons (Reaction 1 below), is generally followed by a water shift reaction to convert CO to H2. The syn-gas (i.e. synthetic mixture of hydrogen and carbon monoxide) produced in the reforming process, can also be used as enriched gas fuel, or converted to liquid fuels such as methanol. Methane CH4 can be reformed with steam or carbon dioxide to form a mixture of carbon monoxide and hydrogen (syn-gas) as follows:CH4+H2OCO+3H2 ΔH=206.2 kJ/mol  (1)CH4+CO22CO+2H2 ΔH=247.3 kJ/mol  (2)where ΔH is the enthalpy of the reaction. At high temperatures (700-1100° C.) and in the presence of a metal-based catalyst, steam reacts with methane to yield carbon monoxide and hydrogen.General Description
One clean fuel production process is solar-driven methane reforming which has been studied extensively [1]. Reaction (2) above can be reversed to produce energy upon demand, to operate in a closed loop, and therefore to provide a means for storage and transportation of solar energy. Reactions with solids such as metal oxides and carbon at high temperature [2-4] provide other solar thermo-chemical cycles for fuel production, without adding CO2 to the environment.
Another example—electrolysis of water—is a simple method for clean fuel (hydrogen) production. However, it has a low attainable efficiency due to the need of using electricity. Recently, Stoots, C. M., O'Brien, J. E., Herring, J. S., Condie, K. G. and Hartvigsen, J. J. “Idaho National Laboratory Experimental Research in High Temperature Electrolysis for Hydrogen and Syngas Production,” Proceedings of the 4th International Topical Meeting on High Temperature Reactor Technology HTR2008, Sep. 28-Oct. 1, 2008, Washington, D.C. USA have suggested performing high temperature electrolysis, possibly using a clean energy source, such as solar radiation. The higher temperature reduces the amount of electricity required for the process.
Yet another method is thermolysis—heating the substance to a temperature where the free energy is equal or larger than zero and it dissociates spontaneously [2]. Although thermolysis of water/steam or carbon dioxide does not require electricity, it requires very high temperatures of above 3000K and 2500K, respectively.
Another clean fuel production process is the photo-catalytic process, which requires neither electricity, nor high temperature. In this process, a high-energy photon initiates an endothermic reaction that produces fuel. However, the efficiency of this method is very low (about 1%) [5-7].
Multi-stage thermo-chemical processes do not require electricity and have practical working temperatures. For example, some thermo-chemical methods of water decomposition can have up to 50% overall heat-to-hydrogen conversion efficiency and operate in medium-to-high working temperatures (T<1000° C.). However, these processes are complex, and handling of rare, expensive and/or corrosive materials is required. Other multi-stage thermo-chemical processes, at higher temperature, e.g. via metal oxide reduction have also been proposed by [4], and more recently by Diver, R. B., Siegel, N. P., Miller, J. E., Moss, T. A., Stuecker, J. N. and James, D. L., “Development of a Cr5 Solar Thermochemical Heat Engine Prototype,” Concentrated Solar Symposium, March 2008, Las Vegas, Nev.
CO2 electrolysis can use different metal electrodes, and liquid or solid polymer electrolytes as shown by [8], and more recently by Stoots, C. M., O'Brien, J. E., Herring, J. S., Condie, K. G. and Hartvigsen, J. J. in the Proceedings of the 4th International Topical Meeting on High Temperature Reactor Technology HTR2008, Sep. 28-Oct. 1, 2008, Washington, D.C. USA. The maximum efficiency of a non-polluting electrolysis system depends on the efficiency of a clean source electricity system, for example, a photovoltaic-driven system. During electrolysis, carbon may deposit on the electrodes, which decreases their efficiency, and eventually stops the process.
Stevens et. al. [8] have shown a current reduction of 40% over 100 min for electrochemical reduction of CO2. According to these experiments, the maximum energy storage efficiency of CO gas (as fuel) was 35%.
Photo-catalytic reduction to CO in high pressure has been investigated by Hori et al. [9] and the direct reduction of CO2 to methane gas was studied by Dey et al. [10]. These processes have low rate reduction and require costly and/or corrosive materials.
There is a need in the art for a novel approach capable of providing an adequate solution for efficient, high rate production of clean and low-cost synthetic fuel.
There is thus provided, according to one broad aspect of the invention, a system for producing one or more compounds with high chemical potential energy, the system comprising: an electron source including a cathode and configured and operable to emit electrons utilizing for example a thermionic (TI) effect; an electric field generator generating an electric field having an energy sufficient to dissociate CO2 and/or H2O reactant gas molecules; and an anode spaced apart from the cathode at a predetermined distance defining a reaction gas chamber configured and operable to cause interaction between the electrons with CO2 and/or H2O gas molecules via a dissociative electrons attachment (DEA) mechanism within the chamber, such that electrons having the required energy dissociate CO2 and/or H2O gas molecules into CO and/or H2 and O2. The reactant gas molecules are therefore at least one of CO2 and H2O and the product compounds are O2 and at least one of CO and H2.
In some embodiments, the electric field generator is exposed to thermal energy emitted from at least one of the electron source and a thermal energy source.
In some embodiments of the invention, the system includes a thermal energy source (heat source) configured and operable to supply thermal energy (radiation) to the electron source thereby raising the electron source temperature and generating thermionic (TI) electrons emission and/or to an electric field generator (e.g. single or plural thermoelectric devices and/or cascades, or a single or plural Stirling Engine) for generating an electric field.
Therefore, in some embodiments, the electric field generator comprises at least one thermoelectric device and/or cascade of thermoelectric devices and operates for utilizing temperature difference generated by the thermal energy source. Alternatively, the electric field generator comprises at least one Stirling engine operating for utilizing temperature difference generated by the thermal energy source.
In some embodiments of the invention, the system includes, instead of just the anode described above, an intermediate electrode adjacent to a gas components separator (e.g. a membrane), both placed in between the anode and the cathode. This configuration enables (a) an additional means of CO2 or H2O dissociation, via electrolysis, and (b) a means for separating between the product compounds of CO and H2 in one side, and O2 in the other side. Therefore, the intermediate electrode is configured and operable to dissociate the reactant gas molecules via electrolysis on the surface of the separator and the gas components separator is configured and operable to separate between O2 and the other product compounds.
The inlet reactant gas is either CO2, or H2O or both. The CO2, and H2O may be introduced into the process on the same side of the separation membrane, or on opposite sides of it. The product compounds exiting the reaction chamber are either CO or H2, or a mixture of both of them. The ions conducted in the membrane are either negative oxygen ions, or protons (H+), or both. Oxygen molecules exit the system on the anode side.
The present invention combines photo, thermal, electric and chemical (PTEC) processes to develop a new method, maximizing the efficiency and the conversion rate of thermal radiation to chemical potential, in the form of CO2 reduction to CO and O2 and H2O reduction to H2 and O2 in the same system. The dissociation of CO2 and H2O may occur in the same system simultaneously or either one of them can be preformed alone. The ratio of CO to H2 is controlled during the process and the mixture of carbon monoxide and hydrogen can be used directly as a synthesis gas (syn-gas) gaseous fuel (e.g. in power or chemical plants), or converted to methanol or other hydrocarbons, which can be used, for example, as transportation fuels. The CO2 and water generated during the burning of these fuels can be trapped, returned to the power plant and reduced again. This method enables clean fuel production on a very large-scale, wherever thermal energy is available.
In some embodiments, the system comprises a gas components separator configured and operable to separate between the oxygen ions and CO and/or H2 molecules resulting from CO2 and/or H2O dissociation. The separator may comprise a membrane configured for allowing only certain gas component such as oxygen ions (O−) to pass therethrough (e.g. transmitting negative oxygen ions). Such membranes may be made of ceramic material, such as for example, Yitria Stabilized Zriconia (YSZ). Its surface facing the chamber containing CO2 has a cathode and the other surface has an anode to extract the electrons from the oxygen ions, attached to a means to transfer these electrons back to the cathode.
In some embodiments of the invention, both the CO2 and the H2O are supplied to the system on the cathode side of the membrane. In this case, the separator is used to separate O− ions from the H2 and CO; it conducts O− ions from the cathode to the anode. The rates of CO2 and H2O dissociation are controlled by the working temperature and by the flow rate of CO2 and H2O entering the cathode side.
In another embodiment of the invention, the CO2 is supplied to the system on the cathode side of the membrane and H2O is supplied to the system on the anode side of the membrane. In this case, the separator can be used to separate H+ ions (protons) from OH−. The same separator can be used to conduct simultaneously O− ions from the cathode to the anode side and H+ ions from the anode to the cathode side. These ion conductions can be done in both directions simultaneously, or in each direction separately. The rates of the O− and H+ ion conductions are controlled by the working temperature and by the flow rate of CO2 on the cathode side and H2O on the anode side.
In another embodiment of the invention, the system includes a thermal energy source (heat source) configured and operable to supply thermal energy (e.g. concentrated sunlight radiation) to the heating elements of at least one Stirling engine, which generates an electric field at a relatively high efficiency.
In another embodiment of the invention, the system includes a separated means of generating the electric field (e.g. a separated solar electric generating system).
The thermal (heat) source may include a solar energy collector, which may for example include a set of reflectors configured to collect sunlight radiation, concentrate it and reflect it towards the electron source.
In some embodiments of the invention, the electron source includes a thermionic cathode or a photocathode. The thermionic cathode may be associated with the electric field generator or a separate electric field generator operable to apply an electric potential onto the electron source, reducing the potential barrier of the cathode and enhancing the number of emitted electrons.
In some embodiments, the thermionic cathode is coated by a protective coating, to be protected from exposure to gaseous environment including CO2, CO, O− and O2. The protective coating may include an oxide metal layer, and may be configured to enable electron transmission via tunneling by reducing the work function of the cathode.
In some embodiments of the invention, the system includes a magnetic field source, operable to adjust the electron motion such that it maximizes the probability of the electron—CO2 dissociative attachment reaction.
In some embodiments of the invention, the CO2 gas is pre-heated by the gases and/or by the hot-side of the reactor walls before entering the reaction chamber.
In some embodiments of the invention, the CO2 gas is excited by exposure to at least one of radiation electron beam (e.g. from a laser source), magnetic field, and electric field, that increases its vibration energy as it enters the reaction chamber. This improves the probability of the electron—CO2 dissociative attachment reaction.
Preferably, the system includes an electron collector configured and operable to collect the emitted electrons, which do not combine with the CO2 molecules.
The system of the invention is operable with high heat-to-chemical potential conversion efficiency, estimated to reach above 40%, and is operable at temperatures in the range of about 600° C.-1500° C.
In some embodiments, the electron source, the electric field generator, the reaction gas chamber and the membrane are integrated in a single module (e.g. cell).
According to another broad aspect of the present invention, there is also provided a system for producing one or more compounds with high chemical potential energy. The system comprises an electron source including a cathode and configured and operable to emit electrons; an electric field generator generating an electric field; an anode spaced apart from the cathode; an intermediate electrode and a gas components separator both placed in between the anode and the cathode; the intermediate electrode being configured and operable to dissociate the reactant gas molecules via electrolysis on the surface of the separator; the reactant gas molecules being at least one of CO2 and H2O, the product compounds are O2 and at least one of CO and H2 respectively.
It should be noted that the system of the present invention provides one or more product compounds having relatively high energy of formation from one or more chemical compounds having relatively low energy of formation. The chemical potential energy of the product compounds can be transformed to other forms of energy such as heat, work or electricity by a chemical reaction.
According to another broad aspect of the present invention, there is provided a method for production of one or more compounds with high chemical potential energy. The method comprises supplying CO2 (e.g. by separating it from other combustion emission gases) and/or H2O reactant gas molecules to a reactor including a cathode an anode and a separator in between the anode and the cathode; applying an electric field between the anode and the cathode having an energy sufficient to dissociate reactant gas molecules via a dissociative electrons attachment (DEA) mechanism and/or to reduce the reactant gas molecules by electrolysis; separating between O2 and the other product compounds molecules; and discharging the product compounds molecules.
The dissociation/reduction of CO2 to CO and O2 and of H2O to H2 and O2 may be carried out as follows: an electron source comprising a thermionic cathode is heated by a heat source to release free electrons therefrom; electrons are emitted from the thermionic cathode using a thermionic (TI) effect; an electric field is generated such as to supply an energy field sufficient to dissociate gas molecules using dissociative attachment effect; introducing the electrons and the gas molecules into a reactor (e.g. reaction chamber), where the electrons dissociate gas molecules to the product compounds.
The heating of the electron source preferably includes supplying thermal energy (e.g. solar radiation) to the electron source thereby raising the electron source temperature and generating thermionic electrons emission from the thermionic cathode. The generation of the electric field may include concentration of the thermal energy and directing it onto an electric field generator.
The thermionic (TI) effect and the electric field generation may be activated by the same thermal energy source, e.g. a solar energy concentrator. The latter may include collection of sunlight radiation, concentration thereof and reflection towards the electron source.
In some embodiments of the invention, gas molecules may be pre-heated before their introduction into the reaction chamber. The pre-heating of gas molecules may be performed using the same thermal energy source operable to activate the thermionic (TI) effect and the electric field generation, for example using at least one heat exchanger.
The number of emitted electrons may be enhanced by applying an electric field to the electron source.
The negative oxygen ions may be conducted through a membrane towards an electron collector; the excess electrons released by the oxygen ions may be combined to form O2 molecules; and the electrons may be recycled back to the electron source. Additionally, the electrons, which did not interact with gas molecules, may also be recycled.
In some embodiments, the electric field may be used to perform electrolysis of the gas (CO2 and/or H2O) on the surface of the membrane, either subsequent to or independent of the dissociative attachment process. The oxygen ions are then conducted through the membrane follow the electrolysis.
The method comprises supplying the CO2 and H2O gas molecules on the same side of the separator, or on opposite sides of the separator.
In some embodiments, the CO2 is introduced on the cathode side of the membrane, while the H2O is introduced on the anode side of the membrane. In this case, the dissociations of CO2 and H2O take place on opposite sides of the membrane and the membrane conducts oxygen ions from the cathode to the anode and protons (H+) from the anode to the cathode.
In some embodiments, the method comprises CO2 trapping by separating CO2 from other combustion emission gases and recycling.
The method may comprise coating at least a part of the thermionic cathode to enable electron transmission via tunneling.
In other embodiments, the method comprises exposing the gas molecules to a radiation or an electrons beam, magnetic, or electric field (e.g. fluctuating field at different orientation) to increase the vibration energy of the gas molecules.